A lateral double-diffused MOS transistor is used in various fields of electrical and electronics engineering, such as power electronics. In this semiconductor device, a high-voltage breakdown characteristic against an overvoltage surge is regarded as the most important, especially when used for switching a current flowing into an inductance load. An outline of a typical construction of a conventional semiconductor device is as follows. Two island shaped N-type regions with high impurity concentration are formed on a N-type layer, which covers a P-type substrate. One of the two island shaped N-type regions is surrounded by a P-type region serving as a base, and plays a role of a source region. The other region is a drain region. A current is controlled by a voltage of a gate electrode separated from a surface of a channel region by an insulating film. The weakest point against a surge voltage in this construction is a curved portion of a P-N junction, where the electric field is concentrated. The withstand voltage of the device can be increased to some extent by suitably selecting a depth and an impurity concentration of an N-layer. Another approach to overcome the aforementioned disadvantage is to add one or two P-type layers to this construction in order to decrease the electric field around the curved portion of the P-N junction, however, a further improvement is extremely desirable.